1. Field of the Invention
The present invention relates to a fixing device mounted in image forming apparatuses, such as electrophotographic copying machines and electrophotographic printers, and a heater used in the fixing device.
2. Description of the Related Art
In recent years, a film heating fixing device mounted in electrophotographic copying machines or electrophotographic printers has been in practical use.
In general, such a film heating fixing device includes a cylindrical film, a plate-like heater that is in contact with the inner surface of the film, and a pressing member that forms a nip portion together with the heater via the film. Since the film heating fixing device can be produced using a low heat-capacity member, the amount of power consumption and the wait time for heating can be advantageously reduced.
However, since the film heating fixing device includes a low heat-capacity member, the temperature of a non-sheet passage area from which heat is not removed by a recording medium easily rises if printing is continuously performed on recording media of a small size. That is, a temperature rise of a non-sheet passage area easily occurs.
To address such an issue, Japanese Patent Laid-Open No. 2005-234540 describes a heater including a heat generating resistor having a positive temperature coefficient (PTC). FIG. 10 illustrates a heater having such a configuration. The heat generating resistor of the heater having a PTC has a conductor electrode portion in each of an upstream section and a downstream section thereof in a direction in which a recording medium is conveyed. An electrical current is applied to the heat generating resistors in the direction in which a recording medium is conveyed. As a result, the electrical resistance of the heater increases with increasing temperature of a non-sheet passage area. Accordingly, heat generation in the non-sheet passage area is reduced and, thus, a temperature rise of a non-sheet passage area is reduced.
In recent years, to further reduce FPOT (first print out time) and power consumption, the size and heat capacity of each of the components of a fixing device have been reduced. Thus, more strict prevention of a temperature rise of a non-sheet passage area is needed. As a result, a heater having an effect to prevent a temperature rise of a non-sheet passage area greater than that of the heater described in Japanese Patent Laid-Open No. 2005-234540 is needed.